1. Field of the Invention
This invention relates to automotive security systems and, more particularly, to automotive security systems that activate an alarm signal when the vehicle is disturbed.
2. Description of the Related Art
Vehicle security systems may be generally described as including five basic elements. They are (1) sensors to detect disturbances to the vehicle; (2) a deterrent subsystem, for example, audible and visual indicators and anti-theft locks; (3) a control mechanism for arming and disarming the security system; (4) a power supply to operate the system; and (5) a control unit that functionally interconnects all of the other elements.
Vehicle security systems are employed in the hope of deterring and preventing persons from damaging or stealing a parked and unattended vehicle, or gaining unauthorized entry. When a vehicle is disturbed, one or more of the sensors are activated, or tripped, and the security system control unit generates an alarm signal. The alarm signal may be used to activate a remote device, such as a pager, or a vehicle-mounted device, such as a siren, flashing lights, an ignition prevention interlock, or some combination of these and other devices.
Many automotive security systems make use of contact switches as the sensors that activate the alarm indicators. Contact switches rely on the relative movement between vehicle panel members, such as movement between a door or trunk lid and the car body. The opening of a door or trunk lid activates the electrical contacts of a switch, which causes the security system to cycle the alarm. Vehicle security system alarm indicators include loud sounds such as sirens and horns, and flashing lights that are usually used in an attempt to frighten off the intruder. Additionally, the alarm signal may disable the vehicle ignition or fuel delivery systems to prevent the vehicle from being driven away. Examples of such systems, often provided by vehicle manufacturers themselves, are shown in U.S. Pat. Nos. 4,612,530; 4,638,292; 4,688,036; and 4,574,,268.
Contact switches are advantageous in that no false alarms are generated. That is, once the security system is enabled by the user, any opening of a door or trunk lid is an unauthorized opening and activates the alarm. A great disadvantage of such contact switches is that the system is insensitive to changes in the environmental conditions that are common with an unauthorized entry. For example, smashing open a vehicle window would not cause activation of a contact switch at the door or trunk.
More effective vehicle security systems have been provided primarily as a result of an improved ability to detect disturbances. This has occurred through the use of increasingly sophisticated sensor technology and the varied methods of responding to the sensors. These security systems make use of sophisticated sensors that are sensitive to disturbances in the environment around and within the vehicle. These environmentally sensitive sensors may be used in place of or in addition to contact switches. The sophisticated sensors, primarily introduced and used by the independent vehicle security industry, include movement or vibration sensors and acoustic sensors.
Motion sensors detect movement of the vehicle that may indicate attempted towing of the vehicle, raising the vehicle for removal of wheels, or the presence of an intruder within the vehicle's passenger compartment, trunk, or engine bay. Acoustic sensors are used primarily to detect breaking glass, such as a vehicle window being smashed open.
Some of these environmentally sensitive sensors, such as the motion sensors and acoustic sensors, are currently being used in vehicle security systems. When the vehicle is moved or a vibration is transmitted through the vehicle to the motion sensor, that sensor is activated, or tripped. Similarly, when a sound in the proper frequency range occurs in the vicinity of the acoustic sensor, that sensor is activated. Upon activation of one of the sensors, the security system in turn generates an alarm, hopefully scaring off an intruder or thief in the process of tampering with the vehicle. Unfortunately, many physical and acoustic "disturbances" to a vehicle are inadvertent or are part of the surrounding environment. These disturbances are sufficient to trigger the environmentally sensitive sensors and therefore the security system, but are not harmful and do not affect the vehicle. Generation of alarms resulting from these events are characterized as false alarms. For example, heavy trucks driving by may cause vibrations that trigger the motion sensor, or police car sirens and honking horns may activate the acoustic sensor.
Other sensors are also known. Ultrasonic transducers may be used to detect, for example, changes in air movement that may indicate a window that has been broken open. Another common form of intrusion sensor detects electromagnetic radiation. The electromagnetic spectrum includes microwave energy, visable light, and infrared radiation. Such intrusion detection sensors are widely employed in premise protection systems, but are rarely used in conjunction with vehicle security systems because of their high false alarm rate.
Presently, electromagnetic intrusion detection sensors are impractical for use in vehicle security systems because of their relatively large size and cost. These sensors are highly sensitive to any environmental change and therefore are also more likely to indicate false alarms than are the previously mentioned sensors. Nevertheless, they are also more likely to detect entry within the vehicle and could "back up" or overlap with other sensors. For example, such sensors can detect air movement within the vehicle as a result of a window being broken open, and could thereby trigger an alarm if the sound sensor malfunctioned or otherwise failed to detect the broken window. As the technology improves, and the detection units associated with the sensors become smaller and more cost effective, their application in vehicle security systems will become practical. Unfortunately, the high false alarm rate associated with these sensors has not been dealt with in the automotive context.
Typically, any single tripped sensor in an automotive security system is sufficient to trigger an alarm. Prior art vehicle security systems therefore cannot utilize the full sensor sensitivity available. A single sensor has a difficult time discriminating between a genuine intrusion or attack and the peak conditions of the ambient environment. All of the sensor types described above have variable levels of detection sensitivity to compensate for the environmental extremes in which they operate. The sensor may have a threshold level of detection much lower than that experienced in the ambient environment. The sensitivity level must be adjusted to a stimulus level much higher than the sensor threshold level of detection in order to reduce false alarms. For example, an acoustic sensor may be capable of detecting very faint sounds, but this capability usually cannot be fully utilized because of the false alarms that would be generated.
The majority of vehicle alarms heard by the public are false alarms. This is due to the sensor's trip threshold, or sensitivity of sensor detection levels, being adjusted below that of peak environmental levels. These decreased threshold sensitivity levels are used in the hope of increasing the sensitivity of detecting disturbances to or tampering with the vehicle. Most false alarms are triggered by activation of a single sensor operating in a single physical medium. True attacks against a vehicle, however, usually involve multiple diverse media, such as the audible sound and physical motion created by breaking a window, or involve a stimulus over an extended period of time, such as the continuous motion caused by towing a vehicle.
In addition, many vehicle security systems use a light or a tone generator constantly pulsing on and off in order to indicate the presense of an alarm system and to ward off potential intruders. The disadvantage of these devices is that small flashing lights cannot be seen well in the light of day and pulsating tones are annoying and keep people awake at night. Therefore, such warning devices are either ineffective or are a nuisance.